The invention relates to a cast aluminum part.
Cast aluminum parts with honed internal cylindrical running surfaces are used in engine manufacture. The cast parts have the following roughness profile as measured according to the German Standard DIN 4776:
R((.sub.k))=0.5 to 3.0 micrometers PA1 R((.sub.nk))=0.1 to 2.0 micrometers PA1 R((.sub.vk))=0.5 to 5.0 micrometers. PA1 R((.sub.k))=0.5 to 3.0 micrometers PA1 R((.sub.nk))=0.1 to 2.0 micrometers PA1 R((.sub.vk))=0.5 to 5.0 micrometers. PA1 R((.sub.k))=0.5 to 2.8 micrometers PA1 R((.sub.nk))=0.1 to 1.8 micrometers PA1 R((.sub.vk))=0.5 to 4.8 micrometers.
Here, R((.sub.k)) is the core or overall roughness, R ((.sub.nk)) the reduced peak height, and R((.sub.vk)) the reduced groove depth.
The internal cylindrical running surfaces, e.g., the cylinder walls of internal combustion engines, have poor wear resistance. Thus, for the most part, these as-cast surfaces are treated before use to improve the wear characteristics.
The treatment can involve galvanic coating of the surfaces. On the other hand, if the cast part consists of a hypereutetic aluminum alloy such as Alusil, the surfaces are etched in order to expose the silicon-containing precipitate of the alloy. The precipitate, which has a markedly better wear resistance than aluminum, then forms the running surfaces.
In areas other than engine manufacture, e.g., tool manufacture, hard coatings are applied to substrates by Plasma Vacuum Deposition (PVD). Examples of the substances used for the coatings are TiN--TiAl--CN--TiCN and CrN. These coatings are very hard and consequently not well-suited for further processing as, for instance, is required for the running surfaces of cylinders.
Another drawback of conventional PVD is that the heat generated causes the substrate to undergo an unwanted thermal treatment. At higher temperatures, e.g., above 500 degrees Centigrade, the substrate may even soften or melt locally resulting in a shape change. In hardenable alloys, heating can change the microstructure to such an extent that the strength is reduced upon cooling.
Adhesion of the coating presents an additional problem. This problem arises because the thermal conductivity of the coating is substantially lower than that of the substrate. If the coated substrate is subjected to temperature fluctuations, the difference in thermal conductivity can cause the coating to loosen.
To avoid the preceding disadvantages, the International Patent Publication WO 89/03930 (Adiabatix) applies a thermally insulating coating to the surface of a cylinder in a thickness of 0.002 inch. This thickness provides a satisfactory insulating effect while improving the adhesion of the coating.